They do have a cdn url for the js and css. You can directly link to those:

https://cdnjs.com/libraries/plottable.js

The page above actually links to the following:

https://cdnjs.cloudflare.com/ajax/libs/plottable.js/2.2.0/pl...

https://cdnjs.cloudflare.com/ajax/libs/plottable.js/2.2.0/pl...